Use your headphones for iPhone calls -- with a Griffin SmartTalk adapter

...The answer is to buy an iPhone headphone adapter, offering an iPhone-compatible mini plug on one end and a button and microphone on the other end, along with a place to plug in whatever headphones (or speakers, or cassette adapters — basically anything with a mini plug) you might have, as well as a clip to attach it to some part of your clothing close enough to your mouth to pick up your end of the conversation...

...The best of the bunch was the Griffin SmartTalk, a real bargain at $20. The SmartTalk’s microphone/clicker module is excellent, a compact block with a small integrated clip. The module was the least obtrusive of the ones I tested, and its button was easy to double-click when I wanted to switch tracks. Its microphone also offered the best sound quality of any of the adapters we tested, providing clear sound with a minimum of noise. The SmartTalk’s cord is wrapped in nylon, which makes it feel a bit classier than the rubber coating on the other adapters.

Besides being half the price of the Bose kit, the SmartTalk works with any headphone set. The Bose kit only works with newer versions of their headphones.

I don't think this would work all that well if the output was going to a car stereo though. Be real tough to avoid feedback with that setup ...